Wait, so fighters finally actually have some effective abilities and you want to nerf THOSE?I thought about something else to include and that is the save mechanics of high level fighters, Champions and the Mage Slayer Feat.
Indomitable becomes a real problem at high level because it is a reroll adding your fighter level on a save, which is almost always going to succeed (I don't think I have seen it fail yet) and high level you can do this up to three times a day. This may not seem like a lot but you only use it on a failed save, so it actually is a lot.
It gets worse though, Heroic Warrior gives the Champion Inspiration every turn in combat, so unless he already spent it to turn a miss into a hit, he is going to get to reroll a failed save before he even has to use Indomitable. Likewise the Mage Slayer feat lets him automatically succeed.
These effects, especially in combination make high level Champion fighters basically immune to any mind control feature (and will make most other fighters nearly immune to). This is a big difference in play as that used to be the weakness you would have your bosses target against them.
I really hate to ban or nerf specific class or subclass abilities but I would strongly consider it for Indomitable and Heroic Warrior.
For ex. indomitable is FINALLY not a joke!
But it's also VERY limited use and gives the fighter some hard choices. So hard disagree from me!